Lower alcohols, such as methanol, ethanol and propanol, are increasingly applied as sustainable transport fuels. The production of these alcohols poses new separation challenges. As an example, the production of ethanol in fermentation reactions results in concentrations of 5-15% of ethanol in an aqueous mixture. Selective removal of the ethanol from this fermentation mixture is difficult, because of the presence of acidic components and fouling agents such as yeast and high-molecular components of the bio-feedstock. Distillation of such mixtures results in ethanol/water mixtures that still contain at least 4% of water, due to the ethanol/water azeotrope. Water/alcohol mixtures of varying ratios, for example containing 1-15% of water, can be purified using water selective membranes.
The state-of-the-art microporous pure silica membranes have shown good separation properties in both gas and liquid separations, but suffer from hydrolysis due to strong interaction with adsorbed water at relevant operation temperatures (95° C. and above). This led to rapid degradation of the microporous structure and loss of selectivity. De Vos et al., 1999; EP-A 1089806) developed hydrophobic silica membranes (also referred to as methylated silica membranes) for separation of gases and liquids and proposed a method for reducing water molecule interaction by incorporation of a precursor containing hydrophobic methyl groups. Methylated silica membranes were further studied for the dehydration by pervaporation of organic solvents by Campaniello et al., 2004. They found that the loss of water selectivity could be retarded by increasing the methyl content (hydrophobicity) of the membranes. Using this approach it was possible to achieve a satisfactory performance up to temperatures of 95° C. However, these membranes are not stable at higher temperatures, which are necessary for efficiently separating water from organic solvents. As a result the observed selectivity decreases, leading to failure within a few weeks at temperatures above 95° C.
Recent work on zeolite NaA and NaY membranes has shown that separation factors ranging from 100-10000 can be achieved with acceptable water fluxes (Ahn, 2006). However, the long-term stability of zeolite membranes under these conditions has not been demonstrated. In contrast, it was shown by Li et al., 2006, that several zeolite membranes such as MOR and MFI do not show stable performance when subjected to hydrothermal conditions. In addition to the limited fluxes, significant reductions of the fluxes were observed in a period of 50 days. Furthermore, it is well-known that the pH range in which zeolite membranes can be applied is limited, because of hydrolytic degradation (Caro J., 2005). Such zeolite membranes are therefore not suited for separating water from alcohol/water mixtures containing acidic components.
More recent investigations have shown that organic-inorganic hybrid silica membranes based on mixtures of the precursors 1,2-bis(triethoxysilyl)ethane (BTESE) and methyltriethoxysilane (MTES) are suitable for the separation of water from several organic solvents, including n-butanol (Castricum et al., Chem. Commun. 2008, 1103-1105; J. Mater. Chem. 2008, 18, 1-10, Sah et al., WO 2007/081212). The long-term stability of these membranes was unprecedented in literature. Membrane life-times up to at least two years were demonstrated at an operating temperature of 150° C. However, subsequent investigations showed that the separation factors of membranes based on BTESE/MTES mixtures in the dehydration of methanol, ethanol, and propanol were disappointing (vide infra).